Sintered molybdenum boron alloy

ABSTRACT

This application relates to a sintered molybdenum boron alloy optionally containing other additives including tungsten, zirconium and hafnium with the boron content of the alloy being less than about 0.2 weight per cent and preferably between 0.005 and 0.01 weight per cent.

United States Patent [191 Benesovsky Aug. 21, 1973 SINTERED MOLYBDENUM BORON ALLOY [75] Inventor:

Austria [73] Assignee: Schwarzkopf Development Corporation, New York, N.Y.

[22] Filed: Oct. 13, 1969 [21] Appl. No.: 865,682

' [30] Foreign Application Priority Data Oct. 11, 1968 Austria A-9933 [52] 11.8. CI. 75/200, 29/182 [51] Int. Cl. B221 1/00, C22c 1/00, C22b 49/00 [58] Field of Search 29/182; 75/176, 200,

[56] References Cited UNITED STATES PATENTS 1,648,679 11/1927 Fonda ..75/207X Friedrich Benesovsky, Reutte, Tirol,

OTHER PUBLICATIONS First Quarterly Report on Investigation of Molybdenum and Molybdenum Base Alloys Made by Powder Metallurgy; Bruckart et al., pp. 6-8 and 22-23 Jaffee, R. 1., Chapter 15, pg. 330354 in The Metal Molybdenum; American Society for Metals, 1956 Primary Examiner-Carl D. Quarforth Assistant Examiner-R. E. Schafer Attorney-Morgan, Finnegan, Durham and Pine [5 7] ABSTRACT 7 Claims, 2 Drawing Figures 5/1957 Green et a1. 75/214 SINTERED MOLYBDENUM BORON ALLOY This application relates to a sintered alloy of molybdenum containing small amounts of boron. Certain molybdenum-boron alloys, particularly with relatively higher boron contents, have been known in the prior art. However, it has not been possible previously to obtain molybdenum-boron alloys by powder metallurgical techniques under circumstances such that the resultant alloy has physical properties at least as satisfactory as molybdenum alone.

It has now been found that under the conditions of this invention molybdenum-boron alloys can be produced to have properties better than molybdenum alone in that they have higher recrystallization temperatures without loss in ductility even in the recrystallized condition. The alloys of this invention exhibit a fine grain structure and as a result, the alloys remain ductile down to temperatures at least as low as 60 C. Nevertheless, the alloys in accordance with this invention have greater tensile strength than unalloyed sintered molybdenum.

The alloys of this invention are made in accordance with powder metallurgical techniques. It is important, however, that the boron content of the alloy be kept very low. The boron content of the alloy will be from about 0.002 to 0.2 weight per cent, preferably less than about 0.01 per cent, generally from about 0.005 to 0.01 weight per cent. In addition, the alloy should contain less than about 0.0025 weight per cent of oxygen.

It is also preferred that the alloy contain a small quantity of tungsten in the range of 0.03 to 0.07 percent by weight, with the tungsten content preferably being about 0.05 percent.

A further improvement in the physical properties of the alloys of this invention can be achieved by the optional addition of one or more of zirconium and hafnium in small amounts. Generally, the presence of a total of from about 0.05 to 2 percent by weight of zirconium and hafnium will be effective.

The alloys of this invention are prepared by powder metallurgical techniques which involves the mixing together of the metallic powders, compression to form a compact of the desired shape followed by sintering and cooling.

The starting metallic powders that are employed should each have a very small oxygen content in order to insure a low oxygen content in the final alloy. Preferably, the oxygen content of the starting powders should not exceed about-350 parts per million. The molybdenum powder. should preferably have a particle size within the range of about 2 to 7 microns. Smallquantities of impurities that do not affect the basic physical properties of the molybdenum itself in the quantities present can be employed but the total impurities should not exceed about 0.2 percent. The boron powder should also preferably have a particle size of from about 2 to 7 microns and it is preferred that the boron be added to the powder mixture in the form of molybdenum boride powder rather than in the form of elemental boron. Elemental boron can, however, be used as a less desirable alternative if it is substantially free of interfering impurities. Naturally, if molybdenum boride powder is used as the source of the boron, the molybdenum portion of the molybdenum boride will contribute to the over-all content of molybdenum in the alloy and calculations must be made accordingly. Some or all of the boron can also be added in the form of other metallic borides in which the metallic component is present in small quantity and does not interfere with the results obtained.

It will often be found during operation that a certain amount of the boron will be lost during sintering, a phenomenon caused by volatilization of the boron oxides formed by reaction of boron with any residual oxygen actually present. However, the properties of the alloy depend on the actual boron content of the end product and not on the boron content of the charged powder. The actual quantity of boron or boron-containing powder to be added and mixed with the molybdenum powder to obtain a predetermined boron content in the alloy will necessarily have to be determined by trial but the trial will be aided by knowledge of the oxygen content of the powders and a rough calculation of the ap proximate quantity of boron oxides that would be expected to be vaporized.

If tungsten is to be added to the alloy, tungsten powder would also be added to the powder mixture and the tungsten powder would also have a particle size preferably within the range of about 2 to about 7 microns and again would be relatively pure and not contain any impurities in quantities sufficient to interfere with the properties of the finished alloy. Tungsten can also be added in the form of a molybdenum-tungsten alloy or mixture Similarly, if zirconium and hafnium are to be added, they can be added in the form of metallic powders but more preferably they are added in the form of zirconium hydride and hafnium hydride powders, respectively. These components would each have a particle size within the range of from about 10 to about 20 microns, preferably having an average particle size of about 15 microns.

Any conventional technique can be used to mix the. powders together such as, for example, ball milling, or

the like. 1

After mixing, the powder mixture is compressed into a compact of the desired shape such as, a rod or bar or particular finished article. The sintering pressure should be in the order of about 4 to 6 tons per square centimeter. The compression can be under ambient temperature conditions.

Following compression, the compacted articles are sintered at an elevated temperature within the range of about l,800 to 2,100 C., preferably 1,900 to 2,000 C. The sintering takes place in a protective atmosphere to prevent oxide formation within the alloy. The protective atmosphere can be an atmosphere of an inert gas such as helium or argon or the like, or it can be a reducing atmosphere such asan atmosphere of hydrogen.- Alternatively, sintering can take place in a protective atmosphere that is substantially evacuated by im: position of strong vacuum. Where vacuum sintering is employed, the minimum vacuum during sintering should be in the range of 10 to 10' mm of mercury. Sintering should take place for a period of time sufficient to achieve adequate alloy fonnation and exact time of sintering will depend on the temperature employed, the thickness of the compact and the exact composition of the mixture. Preferably, a sintering time of from about 2 to 4 hours will suffice. Depending upon the end use desired, the optimum parameters of time and temperature of sintering that are particularly desirable in any given instance can'easily be determined by trial. After sintering has taken place, the sintered alloy should be allowed to cool to room temperature while remaining in a protective atmosphere.

Alloys of this invention containing zirconium and/or hafnium can also be further heat treated by precipitation hardening in accordance with conventional techniques. The mechanical properties of such alloys, particularly their hot strength, can be greatly increased in this manner. Precipitation hardening is preferably achieved by annealing at the temperature in the order of about 1,800 C. followed by quenching to about l,l00 C. and holding at this temperature. This produces coherent precipitates of complex zirconium and- /or hafnium-containing molybdenum phases. The heat treatment requires about 50 to I00 hours.

The molybdenum alloys of this invention, whether or not they contain zirconium and/or hafnium as additives are characterized by particular ease of deformation. For example, recrystallized sheet can be slightly deepdrawn and subjected to other non-machining types of deformation.

The alloys of this invention find particular use in the blades of helium gas turbines of atomic reactor power installations. This utility is possible because of the high creep strength of the alloys. In addition, other utilities for the alloys can be found based upon their good weldability. They can be joined efficiently by electron beam, spot welding or pressure welding. Thus, they can be used in chemical apparatus and as structural parts of electron tubes.

The following examples illustrate the preferred mode of carrying out the invention:

EXAMPLE I A molybdenum powder prepared by reduction and having an average particle size of 5 microns and a specific surface area of 0.2 square meters per gram was employed. This molybdenum powder also had as impurities 60 ppm iron, 30 ppm silicon, 500 ppm tungsten, 100 ppm carbon and 300 ppm oxygen. 1 gram of molybdenum boride were added and mixed with the molybdenum powder, the molybdenum boride having an average particle size of microns. The powder was mixed to fonn an intimate mixture and then pressed into bars at a pressure of 5 tons per square centimeter. The pressed bars were then sintered for 2 hours at 2,000 C. in an atmosphere of hydrogen gas. Following sintering, the bars were allowed to cool to room temperature in the hydrogen atmosphere.

The boron content of the finished alloy was found to be within the range of 50 to 75 parts per million, i.e., in the range of about 0.005 to 0.007 weight per cent.

Similar bars were prepared following the same procedure but containing no boron. Additional bars were prepared containing higher amounts of boron, i.e., bars containing 250 to 350 parts per million boron (about 0.03 percent by weight) and 600 to 750 parts per million boron (about 0.06 percent by weight). Tensile strength and elongation measurements were made of sintered and recrystallized bars at the different levels of boron addition. FIG. 1 shows the elongation displayed by the various sintered and recrystallized bars having different boron additions. FIG. 2 shows the tensile strength in kilograms per square millimeter. Each is shown at varying temperatures within the range of 60 to C. It will be observed that the elongation and tensile strength of the bars of the alloys of this invention at each of the temperatures were significantly better than that of boron-free alloys. It is further observed that the bars containing the lowest quantity of boron yielded the best elongation and that this elongation is achieved without very much difference in tensile strength from the higher boron-containing alloys. Thus, the alloys of this invention, despite their excellent ductility, have a much greater strength than the boron-free alloys and those alloys having less than 0.01 percent by weight of boron are particularly advantageous. This shows that the very small quantities of boron content in the alloys of this invention impart a very substantial benefit to the molybdenum and a more substantial benefit than, in fact, higher amounts of boron.

EXAMPLE 2 The procedure of Example I was followed except that to the powder mixture of molybdenum and molybdenum boride, I percent of hafnium was added in the form of hafnium hydride with mixing and compression into bars at a pressure of 5 tons per square centimeter. The bars were then sintered in hydrogen at 2,l00 C. for 4 hours and then allowed to cool in hydrogen in the furnace. The bars were then annealed for 2 hours at l,800 C. in hydrogen, rapidly cooled, and then again annealed for 50 hours in hydrogen at 1,l00 C. Bars of excellent physical properties were achieved.

EXAMPLE 3 The method of Example I was followed except that in addition to molybdenum and molybdenum boride, additions were made of zirconium hydride and hafnium hydride in powder quantities sufficient to yield 0.5 percent zirconium by weight and 1.5 percent hafnium by weight. All other conditions are the same as in Example I except, in order to attain adequate diffusion of the additions into the base metal, the sintering time was extended for a period of 6 hours.

What is claimed is:

1. In a method of producing a sintered molybdenumboron alloy which has high ductility at low temperature and which contains from about 0.002 to 0.2 percent by weight of boron, which comprises mixing the powdered constituents, compressing the resultant mixture to form a compact, sintering the compact at a sintering temperature for a period of time necessary to achieve sintering and cooling in a protective atmosphere, the improvement which comprises employing molybdenum boride as the source of boron.

2. The method of claim 1 wherein the quantity of molybdenum boride which is employed is sufficient to produce an alloy having from about 0.005 to 0.01 percent by weight of boron.

3. The method of claim 1 wherein the oxygen content of the starting powder does not exceed about 350 parts per million.

4. The method of claim I wherein the molybdenum boride powder has a particle size in the range of about 2 to 7 microns.

5. The method of claim 1 wherein from about 0.03 to 0.07 percent by weight of tungsten is added to the powder mixture before compression.

6. The method of claim I wherein from about 0.05 to 2 percent by weight of an element selected from the group consisting of hafnium and zirconium is added to the powder mixture before compression.

tering temperature in a protective atmosphere for a period of time and at a temperature necessary to achieve sintering, and cooling in a protective atmosphere, the improvement which consists essentially of employing molybdenum boride as the source of boron.

* I II 

2. The method of claim 1 wherein the quantity of molybdenum boride which is employed is sufficient to produce an alloy having from about 0.005 to 0.01 percent by weight of boron.
 3. The method of claim 1 wherein the oxygen content of the starting powder does not exceed about 350 parts per million.
 4. The method of claim 1 wherein the molybdenum boride powder has a particle size in the range of about 2 to 7 microns.
 5. The method of claim 1 wherein from about 0.03 to 0.07 percent by weight of tungsten is added to the powder mixture before compression.
 6. The method of claim 1 wherein from about 0.05 to 2 percent by weight of an element selected from the group consisting of hafnium and zirconium is added to the powder mixture before compression.
 7. In the method of producing a sintered molybdenum-boron alloy containing from 0.002 to 0.2 percent by weight of boron and characterized by high ductility at low temperature which comprises mixing the powdered constitutents, compressing the resultant mixture to form a compact, sintering the compact at a sintering temperature in a protective atmosphere for a period of time and at a temperature necessary to achieve sintering, and cooling in a protective atmosphere, the improvement which consists essentially of employing molybdenum boride as the source of boron. 